In recent years, technologies relating to a separation membrane are being developed, and with characteristics such as space-saving, energy-saving, and filtrate water quality improvement, the technologies are widely used for various purposes such as water treatment. For example, a microfiltration membrane (MF membrane) and an ultrafiltration membrane (UF membrane) are applied to a water cleaning process for producing industrial water or tap water from river water, groundwater, or treated sewage, and applied to pre-treatment and a membrane bioreactor in a seawater desalination reverse osmosis membrane treatment step. A nanofiltration membrane (NF membrane) and a reverse osmosis membrane (RO membrane) are applied to removal of ions, seawater desalination, and a wastewater reclamation process.
Currently, as a method for reclaiming sewage/drainage, for example, there is a method of performing treatment called a “membrane bioreactor (Membrane Bioreactor; MBR)” in which sewage or industrial drainage which has conventionally been treated by an activated sludge process is treated by the MF/UF membrane directly immersed in an activated sludge tank, and performing filtration with the NF/RO membrane installed at the later stage to obtain pure water as product water.
As a system for producing fresh water from seawater or brackish water, there is a technology in which pre-treatment by sand filtration which is a conventional water clarification technology is carried out, and thereafter filtration is carried out with the NF/RO membrane. In addition, there is a method in which seawater or brackish water is pre-treated using the MF/UF membrane as described above, and filtration is carried out with the NF/RO membrane. With this system, since seawater cannot be desalted by the pre-treatment, desalination entirely depends on the treatment with the NF/RO membrane at the later stage.
With the NF/RO membrane separation method, supply pressure being higher than osmotic pressure is desired. Thus, pressure must be applied when supplying raw water to the NF/RO membrane with a pump called a “booster pump”. That is, as a salt concentration of raw water supplied to the NF/RO membrane is higher, the osmotic pressure becomes higher. Thus, it becomes necessary to apply higher pressure with the booster pump, and more energy for allowing the booster pump to operate becomes necessary.
Therefore, in a water producing plant, only one of a sewage/wastewater reclamation process and a seawater desalination process is performed in general. However, in recent years, a membrane treatment system integrating a sewage high-level treatment process and the seawater desalination process has been developed (Patent Document 1, Non-Patent Document 1, Non-Patent Document 2). According to this technology, after sewage is treated by the MBR, fresh water is produced using an RO membrane, and concentrate produced as a by-product at the time of separation of the RO membrane is mixed with seawater. Thus, a salt concentration in the supplied seawater is lowered, and hence the specification of the booster pump in an operation of RO membrane separation used in seawater desalination can more be simplified than in the conventional manner. Therefore, the system becomes further energy-saving.
In such an integrated membrane treatment system using plural kinds of raw water, for example, sewage/drainage and seawater from different supply sources are used as raw water. Thus, feed water quantities thereof are sometimes largely changed from time to time. In particular, sewage/drainage is easily changed due to a human activity time, a plant operation time, or the like. When the respective feed water quantities of raw water are changed, a mixing ratio between the RO membrane concentrate produced as a by-product from a sewage treatment line and seawater is changed. Thus, the salt concentration (osmotic pressure) in seawater to be supplied to the RO membrane on the side of a seawater treatment line is changed.
In a case where although a necessary product water quantity is determined, a sewage/drainage quantity is largely changed, and the product water quantity is ensured by a seawater intake quantity, the salt concentration after mixing is sometimes largely changed. Thus, there is a need for installing a booster pump capable of responding to high pressure to low pressure on the side of the seawater treatment line. When a booster pump having large capacity is controlled and operated by an inverter or the like, efficiency is interior at the time of low pressure, so that an energy-saving effect is decreased. When the mixing ratio of seawater is too high, an effect of diluting the sewage RO membrane concentrate relative to seawater is decreased, and the salt concentration is not really lowered. Thus, there is a problem that an advantage of the integrated membrane treatment system is almost lost. Further, in a case of a fixed mixing ratio, when the feed water quantity of sewage/drainage is decreased, the sewage RO membrane concentrate is unavoidably reduced. Thus, mixing seawater is also reduced, and the feed water to the RO membrane on the side of the seawater treatment line is reduced. Therefore, there is also a problem that the necessary product water quantity cannot be obtained.
In each plant of a sewage plant and other plants, an inflow quantity of sewage and a discharge quantity of drainage are generally determined. In a case where such sewage and drainage are taken as raw water and a collection rate and a mixing ratio of an RO membrane process are uniformly determined, the product water quantity of the system is naturally decided, and the quantity does not always match with user needs. In a case where the sewage/drainage quantity is small but the necessary product water quantity is large, in the system of Patent Document 1, Non-Patent Document 1, Non-Patent Document 2, a concentrate quantity of the sewage/drainage RO membrane process is decreased, and when the seawater intake quantity is increased at a designing stage, there is a problem that an energy-saving effect is small in the system.
Meanwhile, as an example that the sewage high-level treatment and the seawater desalination are performed in one water producing plant, there is a known method of supplying sewage/drainage in a case where sewage/drainage having a lower salt concentration than seawater or brackish water exists as feed water, or supplying seawater or brackish water in a case where sewage/drainage cannot be supplied due to the dry season, stoppage of the plant, or the like (Patent Document 2 and Non-Patent Document 3). According to this technology, sewage/drainage is used as feed water. Thus, the technology is more energy-saving than the seawater desalination using only seawater or only brackish water, and a fixed flow rate of product water can be surely obtained.
In a system in which sewage/drainage is switched with raw water having a different salt concentration (osmotic pressure) such as sewage/drainage and seawater or brackish water to perform filtration with the same RO membrane as in Patent Document 2 and Non-Patent Document 3, the system is often chosen in accordance with the sewage/drainage quantity which is an already determined water quantity.
In recent years, seawater desalination with a reverse osmosis membrane is adopted in a drought-prone region such as the Middle East, and there are a number of large-sized reverse osmosis membrane plants whose water production quantity exceeds 100 thousand m3/d. It is predicted that a necessary water production quantity will be increased and large-sized reverse osmosis membrane plants will be increased in the future.
An enough sewage/drainage quantity can be ensured when a plant is constructed near a large-sized sewage plant and a large plant. However, it is difficult to ensure a large sewage/drainage quantity in a small sewage plant, diversified type sewage plants, and small and middle plants. Therefore, there is a problem that the system of Patent Document 2 and Non-Patent Document 3 cannot cope with a size increase of the system in a case where the necessary water production quantity is large or the sewage/drainage quantity is small.